1. Field of the Invention
The present invention relates to dynamometer control.
2. Description of the Related Art
Dynamometers are used extensively in the testing of internal combustion engines. In speed control of a typical DC dynamometer, closed-loop speed control is effected by feeding back dynamometer speed to the dynamometer controller. Speed is frequently sensed by a sensor having magnetic pickup which senses the rotation of a toothed wheel which rotates with the dynamometer output shaft. The pickup provides both a DC voltage proportional to dynamometer speed and a directional bit, the bit indicating whether the dynamometer is rotating in the forward or reverse direction.
Such a speed detection system can at times be unreliable. The unreliability is sometimes due to the failure of a component in the speed detection system. The unreliability is also sometimes due to the precision required in the installation of the magnetic pickup. If such precision is not present, the speed and direction detection may fail.
A failure in the detection of the speed of a dynamometer can result in the dynamometer going into an overspeed condition. This would result because the dynamometer control strives to keep the dynamometer running at a speed regulation point. If the speed feedback provided by the speed sensor fails, the dynamometer control will believe that the dynamometer is not running fast enough. The dynamometer control will then try feverishly to increase the speed of the dynamometer to reach the speed regulation point. The result will be overspeed of the dynamometer, which can damage an engine coupled to the dynamometer.
Failure of the directional bit of the dynamometer's speed sensor can also have an adverse effect. During an emergency stop of a dynamometer, the dynamometer controller attempts to force the speed of the dynamometer to zero as quickly as possible. This typically entails reversing the polarity of the field voltage of the dynamometer. The directional bit tells the dynamometer controller when the dynamometer has crossed zero speed and is beginning reverse rotation. At this point, the emergency stop procedure is ended. If the directional bit fails, the dynamometer will continue to be driven in reverse. As those familiar with internal combustion engines will attest, driving an automobile engine in reverse has the potential to do great damage to the engine.
As a backup in the case of failure of the directional bit, the speed pickup of the dynamometer can be used to prevent reverse rotation. When the speed pickup is so used, the speed of the dynamometer is sensed during an emergency stop. When the speed reaches a low-speed cutout threshold, the controller ends the emergency stop procedure to prevent reverse rotation of the dynamometer. However, the use of the speed pickup in this way has disadvantages. First, the speed pickup has fundamental reliability limitations, as was discussed above. Further, if the deceleration of the dynamometer is fast enough, the dynamometer can move through zero speed and begin to accelerate into reverse rotation very quickly. Because a typical speed pickup is unipolar, reverse rotation looks the same as forward rotation. If acceleration in reverse occurs quickly, the dynamometer may run above the low-speed cutout threshold in the reverse direction. The controller has no way to distinguish this rotation from proper forward rotation.
In light of the aforementioned limitations in conventional dynamometer control systems, a system which provides improved reliability in preventing overspeed and reverse rotation conditions will provide advantages over the prior art.